Radios with Phase Locked Loops (PLL) are known in the art. Generally, PLL's have a VCO and a loop filter which is used to remove noise components form the control signal of the VCO. The loop filter is switchable between the first state having a wide passband for realizing a large noise bandwidth of the phase locked loop so that a large capture range and a rapid acquisition is obtained and a second state having a narrow passband for realizing a small noise bandwidth of the loop in order that the influence on the oscillator output signal of noise and phase jitter in the offered frequency spectrum can be reduced. An example is shown in the book "Phase Locked Techniques", by F. M. Gardner, John Wiley and Sons, Inc., 1966, page 53.
In time division multiplex (TDM) communication systems where a high speed receiver and transmitter is needed to scan and communicate on the available channels, a loop filter such as one described above may not meet system requirements. The problem arises from the fact that the narrow band filter is not fast enough to locked to the channels as they are being scan. Therefore, a major sacrifice in scan time must be made in order to successfully scan a number of channels. U.S. Pat. No. 4,117,420, DeConinck et al., discloses a PLL having a VCO and an adaptive filter coupled to the control line of the VCO. The loop filter includes two resistors in series with a storage element to allow switching between two states. The switching of the two states occurs at the instant at which the instantaneous value of the signal stored in the loop filter capacitor is equal to the average value of the control signal of the VCO. Furthermore, the switchability is provided via two resistors in series with the storage element. The timing and the method of switching between the two states of the filter renders the output signal of the VCO significantly susceptible to transients. These transients can not be tolerated in fast switching TDM systems. It is therefore clear that a need exists for a radio with a PLL that can successfully and quickly scan a number of channels with minimum transients and without sacrificing stability, noise immunity, and transmission time.